The effect of two platelet inhibitors, naproxen and sulfinpyrazone, on infarct size was studied in 38 open-chest dogs. Seventy-two hours after coronary ligation the heart was sliced and stained with triphenyl-tetrazolium chloride; infarct size, measured by planimetry, was similar among control, naproxen-treated and sulfinpyrazone-treated animals. These results suggest that: 1) platelet inhibition, per se, does not affect infarct size and 2) the infarct-sparing action of ibuprofen (a drug similar to naproxen) may be due to properties other than those common to naproxen.